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ADC Input Type Question and Part Recommendation

JSly
Intellectual 410 points
Other Parts Discussed in Thread: ADS8691, ADS8860

Hello,

I'm trying to select an ADC right now and I'm a bit confused by all the different ADC input types and which is the right one for me. The signal that I'm converting is coming from the LA 55-P current transducer. It's a bipolar signal and will swing from +5V to -5V. Do I need single ended bipolar input? If you could shed some light on this, it would be much appreciated.

Also, if you could provide an ADC recommendation, that would be much appreciated as well. The requirements are as follows

  • 1 input channel
  • Sample rate of 1MSPS or higher
  • >12 bits of resolution
  • Less parts, the better. Faster assembly and lower elements in the signal chain (lower error) 
  • ADCs need to be able to be daisy chained
  • Enhanced SPI would be preferred. We need to transmit signals over a cable so the lower speed signals would be preferred
  • No BGA or LGA. We need to solder these boards by hand. QFN is doable, but not preferred. 

I was looking at the ADS8691. Would this be a good fit? Is the input type correct?

I am by no means an expert in analog design so if I'm misunderstanding anything in these requirements, please don't hesitate to point it out. I would appreciate that.

Thanks!

  • 0
    TI__Mastermind 36230 points

    Hi Jason,

    You have a bipolar signal since the voltage span includes positive and negative voltages.  Because the sensor output is basically single ended (The burden resistor is connected to GND), a single ended ADC input will work well.  Please note that any ADC input configuration can be used with proper external signal conditioning.

    I suggest you check out TI Precision Labs - ADCs which covers several analog topics related to ADC's.

    https://training.ti.com/ti-precision-labs-adcs

    For a detailed explanation of the different ADC input types, take a look at Section 2.1.

    https://training.ti.com/ti-precision-labs-adcs-sar-adc-input-types?cu=1128375

    The ADS8691 will work, but since it has an integrated analog front end, the input signal bandwidth is limited to 15kHz for this device.  I assume you want to support the full bandwidth of the LA 55-P, DC-200kHz.  If this is the case, you will want to use a different ADC.

    The following Analog Engineer's Circuit shows how to convert a +/-10V input to drive a single-ended input ADC (ADS8860, 1Msps, 16b).  The resistor values used to scale the +/-10V down to a 0->5V range can easily be adjusted for a +/-5V input range.  This circuit topology will support the full signal bandwidth of your current sensor.

    http://www.ti.com/lit/an/sbaa374/sbaa374.pdf

    Regards,
    Keith N.
    Precision ADC Applications